Method for producing cold rolled steel plate of super high strength

Abstract

The present invention provides a method for manufacturing an ultra high strength cold-rolled steel sheet, comprising the step of continuously annealing a cold-rolled steel sheet consisting essentially of, in terms of weight percentages, 0.07 to 0.15% C, 0.7 to 2% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, 0.0003 to 0.003% B, and the balance being Fe, in which such continuous annealing comprises the steps of: heating the cold-rolled steel sheet at from 800° C. to 870° C. for 10 seconds or more; slowly cooling the heated steel sheet down to from 650° C. to 750° C.; rapidly cooling the slowly cooled steel sheet down to 100° C. or less at a cooling speed of over 500° C./sec; reheating the rapidly cooled steel sheet at from 325° C. to 425° C. for from 5 minutes to 20 minutes; cooling the reheated steel sheet down to room temperature; and coiling the cooled steel sheet. According to the invention, the ultra high strength cold-rolled steel sheet, for use in a structural member of automobile, which has a tensile strength of 980 MPa or more and is excellent in stretch-flangeability, ductility, and spot-weldability can be obtained.

1. A method for manufacturing an ultra high strength cold-rolled steel sheet, said steel being a ferrite martensite dual-phase steel, the method comprising continuously annealing a cold-rolled steel sheet consisting essentially of, in terms of weight percentages, 0.07 to 0.15% C, 1.4 to 2% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol.Al, 0.005% or less N, 0.0003 to 0.003% B, optionally at least one element selected from the group consisting of Ti and Mo, and the balance being Fe,
 wherein said continuous annealing comprises the steps of: 
 (a) heating the cold-rolled steel sheet at from 800° C. to 870° C. for 10 seconds or more to provide a heated steel sheet; 
 (b) slowly cooling the heated steel sheet down to from 670° C. to 750° C. to provide a slowly cooled steel sheet; 
 (c) rapidly cooling the slowly cooled steel sheet down to 100° C. or less at a cooling speed of over 500° C./sec to provide a rapidly cooled steel sheet; 
 (d) reheating the rapidly cooled steel sheet at from 325° C. to 425° C. for from 5 minutes to 20 minutes to provide a reheated steel sheet; 
 (e) cooling the reheated steel sheet down to room temperature to provide a cooled steel sheet; and 
 (f) coiling the cooled steel sheet, 
 thereby obtaining a steel sheet having a tensile strength of 980 MPa or more, a hole-expanding ratio of 60% or more and an elongation of 15% or more. 
 
     
     
       2. The method of manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , further containing at least one element selected from the group consisting of, in terms of weight percentages, 0.003 to 0.03% Ti and 0.1 to 1% Mo. 
     
     
       3. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein step (b) is carried out at a cooling speed of less than 20° C./seconds. 
     
     
       4. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein step (b) is carried out at a cooling speed of 5° C./second to 15° C./second. 
     
     
       5. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , further comprising before the continuous annealing, producing a steel sheet by a continuous casting method or an ingot making method, cooling the steel sheet, reheating the steel sheet, hot-rolling the steel sheet and cold-rolling the steel sheet, wherein in the hot-rolling, a finish rolling temperature is from the Ar 3  transformation temperature to 870° C. 
     
     
       6. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , further comprising after the continuous annealing, temper-rolling the steel sheet at a rolling reduction rate of 0.1 to 0.7%. 
     
     
       7. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein the steel sheet contains 0.12 weight % of said C, 2.1 weight % of said Mn, 0.010 weight % of said P, 0.003 weight % of said S, 0.030 weight % of said Sol.Al, 0.003 weight % of said N, 0.0010 weight % of said B, less than 0.001 weight % of said Ti and less than 0.001 weight % of said Mo. 
     
     
       8. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein the steel sheet has a composition of 0.13 weight % C, 1.4 weight % Si, 1.9 weight % Mn, 0.010 weight % P, 0.003 weight % S, 0.030 weight % Sol.Al, 0.003 weight % N, 0.0010 weight % B, less than 0.001 weight % Ti, less than 0.001 weight % Mo, and the balance being Fe. 
     
     
       9. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein the steel sheet contains 0.12 weight % C , 2.2 weight % of said Mn, 0.010 weight % of said P, 0.003 weight % of said S, 0.030 weight % of said Sol.Al, 0.003 weight % of said N, 0.0010 weight % of said B, 0.02 weight % of said Ti and less than 0.001 weight % of said Mo. 
     
     
       10. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein the steel sheet contains 0.12 weight % of said C, 1.9 weight % of said Mn, 0.010 weight % of said P, 0.003 weight % of said S, 0.030 weight % of said Sol.Al, 0.003 weight % of said N, 0.0010 weight % of said B, 0.30 weight % of said Mo and less than 0.001 weight % of said Ti. 
     
     
       11. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein the steel sheet further contains 0.003 to 0.03 weight % Ti. 
     
     
       12. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein the steel sheet further contains 0.1 to 1 weight % Mo. 
     
     
       13. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein in step (a), the cold-rolled steel sheet is heated at a heating speed of about 20° C./second, at 830° C. for 300 seconds; in step (b), the heated steel sheet is cooled to 700° C. at a cooling speed at about 10° C./second; in step (c), the slowly cooled steel sheet is rapidly cooled in water at a cooling speed of about 2000° C./second; and in step (d), the rapidly cooled steel sheet is subject to reheating at 400° C. for 10 minutes. 
     
     
       14. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 7 , wherein in step (a), the heating is carried out at 830° C. for 150 seconds; in step (b), the slowly cooling is carried out at a cooling speed of 10° C./seconds; in step (c), the rapidly cooling starts at a temperature of 720° C. and is carried out at a cooling speed of 2000° C./second; and in step (d), the reheating is carried out at a temperature of 400° C. for 6 minutes. 
     
     
       15. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 8 , wherein in step (a), the heating is carried out at 830° C. for 120 seconds; in step (b) the slowly cooling is carried out at a cooling speed of 7° C./seconds; in step (c), the rapidly cooling starts at a temperature of 700° C. and is carried out at a cooling speed of 2000° C./seconds; and in step (d), the reheating is carried out at a temperature of 405° C. for 10 minutes. 
     
     
       16. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 7 , wherein in step (a), the heating is carried out at 840° C. for 160 seconds; in step (b), the slowly cooling is carried out at a cooling speed of 21° C./seconds; in step (c), the rapidly cooling starts at a temperature of 725° C. and is carried out at a cooling speed of 2000° C./second; and in step (d), the reheating is carried out at a temperature of 410° C. for 15 minutes. 
     
     
       17. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 9 , wherein in step (a), the heating is carried out at 840° C. for 100 seconds; in step (b), the slowly cooling is carried out at a cooling speed of 10° C./seconds; in step (c), the rapidly cooling starts at a temperature of 670° C. and is carried out at a cooling speed of 2000° C./second; and in step (d), the reheating is carried out at a temperature of 410° C. for 9 minutes. 
     
     
       18. The method for manufacturing an ultra high strength cold-rolled steel sheet as set forth in  claim 1 , wherein C is in an amount of 0.12 to 0.15 weight %.